Fusion draw process is used to make a sheet of material from molten material such as molten glass (Dockerty U.S. Pat. No. 3,338,696 and Dockerty U.S. Pat. No. 3,682,609). Typically, the fusion draw process involves delivering molten material into a trough and overflowing the molten material down the sides of the trough in a controlled manner. The separate streams of material flowing down the sides of the trough merge at the root of the trough into a single stream of material, which is drawn into a continuous sheet of material. The continuous sheet of material is separated into discrete pieces at the bottom of the fusion draw machine. A key advantage of this process is that the surfaces of the sheet of material do not come in contact with the sides of the trough or other forming equipment and therefore are pristine. Another benefit of the process is that the sheet of material is very flat and has a uniform thickness (Dockerty U.S. Pat. No. 3,682,609).
Large sheets of glass produced by fusion draw process are a key component in making large flat panel displays. Alternatively, they can be diced to make other devices such as active electronic devices, photovoltaic devices, and biological arrays. However, as the demand for larger large-sized sheet increases, so does the difficulty in forming and handling of these sheets. For example, sheet scoring and separation processes at the bottom of the fusion draw machine contribute significantly to the sheet motion in the forming zone of the fusion draw machine. Sheet motion in the forming zone can negatively impact the level of stress and stress variation within the sheet, possibly leading to distortion in the final product. The larger the sheet being handled, the more significant the effect of sheet motion can be on the stress level and variation with the sheet.
Corning Incorporated, the assignee of the present invention, has developed various techniques for minimizing sheet motion at the bottom of the draw. One such technique involves scoring the glass sheet by laser, thereby avoiding physical contact with the glass sheet that can result in sheet motion (Abramov et al. U.S. patent application Ser. No. 12/008,949). Another technique involves use of a conformable nosing device to engage a glass sheet while the glass sheet is being scored, thereby reducing motion of the glass sheet during scoring (Chalk et al. U.S. Patent Publication 2008/0276646). Another technique involves separation of the glass sheet without bending the glass sheet (Kemmerer et al. U.S. Patent Publication US 2007/0039990). These techniques require real-time information about the displacement and shape of the glass sheet. Such information at different elevations of the FDM may also be useful in fine-tuning and optimizing the draw process.
Smooth glass sheets have surfaces that behave as specular reflective surfaces for visible light. Shape measurement of specular reflective surfaces by optical means is fundamentally different from shape measurement of diffuse reflective surfaces by optical means. A diffuse reflective surface can be considered as a collection of secondary point light sources. Thus, the shape of a diffuse reflective surface may be estimated by locating the position of these sources. A specular reflective surface, on the other hand, cannot be observed directly. Only reflection from the specular reflective surface is visible. The problem of measuring the shape of a specular reflective surface has been studied in, for example, Savarese et al., “Local shape from mirror reflections,” International Journal of Computer Vision, 64(1), 31-67 (2005); Haeusler, et al U.S. Patent Publication 2005/0238237; Knauer et al., “Phase measuring deflectometry: a new approach to measure specular free-form surfaces.” In Optical Metrology in Production Engineering. Proceedings of SPIE v. 5457 (2004): 366-376.; Kochengin et al, “Determination of reflector surfaces from near field scattering data.” Inverse Problems v. 13 (1997): 363-373, and Winkelbach, et al “Shape from single stripe pattern illumination.” Ed. Luc Van Gool. In Pattern Recognition. Lecture Notes in Computer Science v. 2449 (Springer, 2002), 240-247. These references did not study the problem of measuring the shape of a large-sized glass sheet, such as useful in the flat panel display industry.
Techniques for measuring shapes of specular reflective surfaces have the same difficulty to overcome: slope-position uncertainty. The slope-position uncertainty problem can be illustrated with reference to FIG. 1 (Haeusler et al. U.S. Patent Publication 2005/0238237). In FIG. 1, a camera K1 captures a reflection of a pattern 2 via a specular surface 3. Line 5a represents a beam coming from point 7 on a screen 1, where pattern 2 is produced, and incident on point 6 on the specular surface 3. Line 5b represents a beam reflected from point 6 on the specular surface 3 and incident on point 9 in the image plane 8 of the camera K1. The positions of screen 1 and camera K1 are known. The positions of point 7 and point 9 are also known. However, this information is not sufficient to allow the position of point 6, having surface normal 11, to be determined with certainty for two reasons: (i) the specular surface 3 is invisible and (ii) other points along the line of sight 5b, e.g., point 6a having suitable surface normal 11a, would also image point 7 to point 9. Without knowing the position of reflection points on the specular surface, it is not possible to uniquely determine the shape of the specular surface.
Haeusler et al. U.S. Patent Publication 2005/0238237 and Knauer et al. “Phase measuring deflectometry: a new approach to measure specular free-form surfaces.” In Optical Metrology in Production Engineering. Proceedings of SPIE v. 5457 (2004): 366-376, use stereo-deflectometry to resolve ambiguities in position of the reflection point. The method generally involves capturing multiple reflected images of a sinusoidal pattern from different lines of sight and looking for points in the measuring space at which potential surface normals have the least deviation from one another. Kochengin, et al “Determination of reflector surfaces from near field scattering data.” Inverse Problems v. 13 (1997): 363-373, takes a different approach including measuring the shape of the reflecting surface R from near-field scattering data measured on an object T. The setup is such that rays reflected off the reflecting surface R are incident on the object T. The position of object T is known, Kochengin, et al. “Determination of reflector surfaces from near field scattering data.” Inverse Problems v. 13 (1997): 363-373 and shows that if the position and intensity of source O are also known, the reflector can be determined by solving an inverse problem. Savarese et al., “Local shape from mirror reflections,” International Journal of Computer Vision, 64(1), 31-67 (2005) propose schemes for measuring local geometric information of a mirror surface around a reflection point r by analyzing the deformation produced upon a planar pattern of intersecting lines through specular reflection on the mirror surface at the point r.